If you're training for a distance race, you're probably familiar with the market of sports beverages promising to hydrate and fuel your run better than the next guy's stuff. Gu, Gatorade, Nuun—no matter where you look, all of a sudden you're being told that pure water won't cut it.

Trying to figure out what your body needs and when can be seriously confusing. That's why we did some digging for you.

Here, top exercise physiologists, hydration experts, and coaches share what they want you to know about staying hydrated during your long runs (and why water really isn't enough).

Athletes Need Sodium

There's a lot of science surrounding endurance hydration, but put very simply, it boils down to this: "Water isn't enough, and plain water can actually slow down fluid absorption," says Stacy Sims, Ph.D., an exercise physiologist and nutrition scientist who specializes in hydration. Sodium, in particular, works to help your body absorb fluids such as water, keeping you hydrated, she says. "You need sodium to activate certain transport mechanisms across the intestinal cells into the blood."

Also, since you lose sodium via sweat, if you exercise for more than about two hours and only drink water, you risk diluting your blood's sodium concentration, explains Corrine Malcolm, an ultrarunning coach at Carmichael Training Systems. This can lead to something called hyponatremia, which is when sodium levels in the blood are too low. Plus, symptoms of the condition can actually mimic signs of dehydration—nausea, headache, confusion, and fatigue, she says.

But because sweat composition and perspiration rates vary from person to person, it's hard to say how much sodium you need during an endurance event, says Sims.

Generally, Malcolm suggests about 600 to 800mg of sodium per liter of water and 16 to 32 ounces of water an hour during exercise that lasts longer than an hour. Products with 160 to 200mg of sodium per 8-ounce serving are also good bets, adds Sims.

The good news is that you don't need to immediately replace *all* the sodium you lose during a workout. "The body has plenty of sodium stores," says Sim. "As long as you are eating and drinking foods with sodium in them, you are providing what your body needs, as it needs it." (Note: Iodine Deficiency Is On the Rise Among Fit Women)

Working with a registered sports dietitian can also help you zero in on what's best for you.

The Science of Hydration

Another often overlooked issue regarding hydration has to do with osmolality, which is just a fancy way of saying "the concentration of whatever you're drinking," says Malcolm.

A little physiology crash course: Your body utilizes osmosis—the movement of fluid (i.e. blood, water, or a digested sports drink) from an area of low-concentration to one of high-concentration—to transport water, sodium, and glucose, she says. When you eat or drink something, the nutrients your body needs are absorbed by the GI tract into your body. The problem? "Sports drinks that are more concentrated than your blood will not move from your GI tract to the body and will instead pull fluid out from cells, causing bloating, GI distress, and ultimately dehydration," says Malcolm.

To promote hydration, you want a sports drink that is less concentrated than your blood, but higher than 200 mOsm/kg. (In case you wanna get all pre-med biology with it, blood osmolality ranges from 280 to 305 mOsm/kg.) For sports drinks, which provide carbs and sodium, aim for an osmolality between about 200 and 250 mOsm/kg. If you're wondering how in the world you're supposed to know how much osmolality a beverage has, well, it's tricky, but there are a couple ways you can find out (or make an educated estimate). Some companies list these values, though you might have to dig a bit to find them. Nuun Performance has 250 mOsm/kg, a figure you can find on their website. You can also gauge osmolality by looking at ingredients and nutritional breakdown on the label. Ideally, you want no more than 8g total carbohydrates per 8 ounces with a mix of glucose and sucrose, says Sims. If possible, skip fructose or maltodextrin as these don't help the body absorb fluids.

Pre- and Post-Workout Hydration

Drinking before and after a workout helps maintain your body's happy state of equilibrium. "Going into your runs well-hydrated helps you not only feel better but also mitigates loss that you naturally expect to happen during exercise," says Malcolm. (Related: Best Pre- and Post-Workout Snacks for Every Workout)

Often, the best pre-run hydration simply involves practicing good hydration throughout the day (read: not downing a huge bottle of water 10 minutes before your run). Check the color of your pee to see if you're on the right track. "You want it to look more like lemonade and less like apple juice during the day," says Luke N. Belval, C.S.C.S., director of research at UCONN's Korey Stringer Institute. "You don't want your urine to be clear as that indicates overhydration."

Post-workout, watery fruit and veggies, or salty soups can help restore lost sodium, suggests Sims. Look for ways to get more potassium, too. "It's the key electrolyte for post-exercise rehydration," says Sims. Sweet potatoes, spinach, beans, and yogurt are all good sources. "One of the best dehydration replacement methods is chocolate milk," says Belval. "It contains fluids, carbohydrates, proteins, and some electrolytes."

You could also consider supplementation throughout the day. Nuun offers dissolvable tablets you can drink in water throughout the day.

A good test to see whether you might want to consider electrolyte supplementation? "See if you have any salt deposits on your clothes after working out. This might indicate you're a salty sweater," says Belval.

Just remember the golden rule of training: Don't try anything new on race day. Test out your hydration (as well as any nutrition changes) before, after, and during long runs, then check in with yourself: Did you notice a dip in energy or mood? Did you pee during your run? What color was it?

"It's important to look at how you feel," reminds Malcolm. "Making mistakes is part of racing, but making the same mistakes over again is avoidable."